There are different types of chemical reactions. Among the most common are hydrolysis and dehydration. In addition to their experimental and industrial use, these two reactions are important in biological systems, especially in metabolic activities.
The difference between hydrolysis and dehydration is that hydrolysis is the breakdown of larger molecules into smaller ones (lysis) whereas, dehydration is the removal of water with the help of a dehydrating agent, to yield a new product. Usually, it is termed dehydration condensation.
Hydrolysis vs Dehydration
| Hydrolysis | Dehydration |
| Hydrolysis is the breakdown of larger molecules into smaller ones upon addition of water | Dehydration is the removal of water molecules with the help of dehydrating agents. |
| Hydrolysis is the Greek word, “hydro” means water and “lysis” means breakdown | Dehydration also has an ancient Greek origin, “de” means removal of and “hydro” is for water |
| Water molecule(s) are added | Water molecule(s) are removed |
| Hydrolysis is a decomposition reaction | Dehydration is a combination reaction |
| The products of hydrolysis are simple molecules | The products of dehydration are complex molecules |
| No byproduct is formed | Water molecules are the byproduct |
| Digestion of proteins, lipids, and carbohydrates in biological systems are examples of hydrolysis | Dehydration of alcohols with dehydrating agents such as sulfuric acid to form ethers are examples of dehydration |
What is hydrolysis?
The word hydrolysis comes from Greek “hydro” and “lysis” meaning ‘water’ and ‘breakdown’. Hydrolysis is the breakdown with the help of water. Any molecule upon gaining water molecule(s) and breaking down into its parts is said to be hydrolyzed. However, not all bonds can be hydrolyzed.
Hydrolysis is the bond-breaking and energy-releasing reaction. It is an important biological reaction that involves the release of energy inside our bodies. The food we eat daily is composed of complex molecules of proteins, carbohydrates, lipids, etc which are broken down into simple molecules by various enzymes. The energy released during this reaction is stored in the form of ATP (Adenosine triphosphate), the energy currency of our body. This stored energy is later used for biochemical metabolic processes.
Examples of the hydrolysis reaction
- Hydrolysis of salts
A common example is the hydrolysis of salt of weak acids or bases. Due to the structure of such formula compounds and their polarity, water begins to ionize into hydroxide anions and hydronium cations. The salt also forms its constituent cations and anions which interact with water ions to form different compounds.
For example, sodium acetate (C2H3O2Na) hydrolyzes to form sodium hydroxide (NaOH) and acetic acid (CH3-CO2H).
- Acid-base catalyzed hydrolysis
Hydrolysis driven by the presence of acid or base is termed acid-base catalyzed hydrolysis. Acids protonate carbonyl oxygen (in case of aldehydes, ketones, esters, amides, etc), increasing their susceptibility to be attacked by a nucleophile e.g. a water molecule. Bases, on the other hand, attack the electrophilic carbonyl carbon, inducing the hydrolysis reaction to take place. This type of hydrolysis reaction yields carboxylic products in the end.
For example, hydrolysis of esters, hydrolysis of amides, etc are acid-base catalyzed hydrolysis.
- Hydrolysis of sugars
Hydrolysis of sugar is called saccharification. Upon hydrolysis, sucrose breaks down into glucose and fructose.
- Hydrolysis in biological systems
Hydrolysis in biological systems is catalyzed by enzymes. One good example is the hydrolysis of ATP. Digestion of proteins, lipids, and carbohydrates, etc are also examples of hydrolysis in biological systems.
What is dehydration?
Dehydration is often called dehydration synthesis. Dehydration synthesis is a process in which a molecule is synthesized and a water molecule(s) is removed as the name suggests. It is termed condensation reaction only when the small products being released are necessarily water molecules.
Sulfuric acid (conc.) is the dehydration agent most commonly used. The temperature limit for this reaction is 130 °C to 140 °C at high pressure. This reaction is termed Williamson’s synthesis of ethers.
Dehydration can be done in two ways:
The first is to remove water molecules from one substance yielding an unsaturated bond. This can be done by protonating hydroxyl (OH–) to OH2+. The OH2+ is a good leaving group thus, the water molecules leave easily. However, a dehydrating agent is required for this reaction. Commonly used dehydrating agents are concentrated sulfuric acid, alumina, concentrated phosphoric acid, etc.
The second way is to bring two separate molecules together and remove hydroxyl (OH–) from one and proton (H+) from another, thus, condensing them into one large molecule. This method is important for organic synthesis, including ester synthesis, amide synthesis, and aldol condensation. The biosynthesis of molecules in biological systems is governed by this method.
Examples of dehydration
- Fischer esterification
The most common example of dehydration is Fischer esterification. It is a widely used industrial method to produce esters.
- Disaccharide and polysaccharide formation
Two monosaccharides can be combined to form a disaccharide and remove a water molecule. For example, fructose and glucose join to form sucrose. Similarly, disaccharide joins to form polysaccharides, etc.
- Dehydration of Alcohols
Alcohols react with protic acids to form alkenes by losing a molecule of water. These reactions are known as dehydration of alcohols. Dehydration of such type is also known as dehydrogenation reaction. They are also considered elimination reactions.
For example, ethanol after going through a dehydration reaction gets converted into ethene.
Hydrolysis vs. Dehydration Synthesis Overview
Concepts Berg
What are the similarities between hydrolysis and dehydration?
- Both hydrolysis and dehydration are catalyzed by specific enzymes.
- They are important reactions of biological systems.
- They are both important in organic synthesis reactions.
Does hydrolysis release water?
In hydrolysis, water combines with reactants to form products. Dehydration, however, releases water as a by-product.
Is lipid synthesis hydrolysis or dehydration?
Lipids are macromolecules that get formed in dehydration synthesis reactions.
How do dehydration synthesis and condensation differ from each other?
Generally, dehydration synthesis is referred to as condensation reactions because condensation reactions usually release water molecules as by-products. However, if during condensation, any product other than water is produced, it is not a dehydration reaction anymore.
As a general rule, all dehydration reaction reactions are condensation reactions while all condensation reaction reactions are not dehydration synthesis.
What is hydration energy?
Hydration energy is the amount of energy released when a lattice is broken down into ions by solvation in water.
For example, the dissolution of NaBr in water, dissolution of NaCl in water, etc.
Why are polymerization reactions also referred to by the name of dehydration synthesis reactions?
Polymerization is defined as the combination of small units called monomers into long-chain units called polymers. This process is a condensation reaction unless water is eliminated as a by-product. Therefore, polymerization is sometimes referred to as “dehydration synthesis” reactions.
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